The rapid growth of the subscriber base in GSM (Global System for Mobile communications) has stressed the need for increased voice capacity. Thus, both mobile network operators and telecom equipment manufacturers have agreed to open a new study item in the 3GPP standardization. The study item has been named MUROS and is described in GP-072027, “WID on MUROS”. The basic idea is to allow two users to share the same frequency band and the same time slot, both in the downlink and in the uplink. There are several possible technical realizations of MUROS. One proposed technique is to use a hybrid quadrature modulation in the downlink channel. The two user signals are mapped to the real and imaginary parts of the baseband signal. These are called the I- and Q-sub-channels, and under some conditions they are orthogonal, and therefore named OSC (Orthogonal Sub-Channels). The technique can be made to include modulation of data transmitted using a QPSK modulation scheme in a cellular radio system to two mobile stations multiplexed on a shared channel comprising two branches, such that the total energy of the QPSK modulated is divided unequally between the two branches of the modulated signal resulting in that an improved radio system can be obtained. The modulation with energy divided unequally between the two branches of the modulated signal can be termed hybrid quadrature modulation. Such a modulation method is further described in the co-pending international patent application No. PCT/SE2008/050116 incorporated herein by reference.
In the hybrid quadrature modulation, the symbol constellation is in quadrature, with the 4 symbols lying on the unit circle in the complex plane. The orthogonality of the I and Q branches is preserved. However, a cross power branch ratio parameter α is introduced, allowing the total energy of the signal to be divided unequally between the two sub channels. This parameter α may be changed over time. The parameter may for example be changed from one transmission time slot to the next transmission time slot. It is chosen so that 0≦α≦1 or (0≦α≦√{square root over (2)}) In the extreme case when α=1 the power is divided equally between the I/Q branches, resulting in ordinary QPSK modulation. When α=0 all the signal power is given to one of the branches yielding BPSK modulation. Other values of α causes intermediate distributions of the total energy between the I and Q sub channels. In accordance with one embodiment, the parameter α can be chosen adaptively, for example based upon feedback from one or both mobile stations receiving data via the shared downlink channel, or using a fixed scheme. Thus, the user data is modulated using a quaternary symbol constellation where the data is transmitted to two mobile stations multiplexed on a shared channel comprising two branches, where the branches correspond to the real and imaginary parts of one complex-valued baseband signal.
This allows compatibility between MUROS capable networks and legacy mobile stations.
Even though legacy mobiles are supported by the hybrid quadrature modulation described above, new mobiles will still be required because a new training sequence set is introduced.
The hybrid quadrature modulation can employ a time varying symbol constellation called adaptive α-QPSK. This quaternary constellation is parameterized by a real-valued parameter α. This parameter defines the shape of the symbol constellation, and it can change from burst to burst. Thus, in theory, there can be an infinite number of different symbol constellations. However, in some embodiments the total number of possible symbol constellations is finite. Adaptive α-QPSK has in one embodiment been designed so that the mobile station receiver need not know the shape of the constellation in order to successfully demodulate and decode the received signal. The two sub-channels can be separated by means of orthogonal training sequences. A traditional receiver might employ SAIC (Single Antenna Interference Cancellation), and it will only need to know its own training sequence in order to demodulate and decode its sub-channel. Such a receiver is depicted in FIG. 2.
A problem with a conventional receiver is that the energy contained in the other orthogonal sub-channel (i.e. the sub-channel intended for the other user) constitutes a source of interference and it is not an aid but rather a hinder to the successful demodulation of the desired signal.
In 3GPP GP-070214, “Voice Capacity Evolution with Orthogonal Sub Channel”, source Nokia,
it is proposed that new mobiles might have knowledge of the training sequence of the other sub-channel and use it in order to improve the equalization of the desired signal. However, this approach cannot be used if hybrid quadrature modulation is employed since the shape of the symbol constellation may not be known. In other words, knowledge of the two training sequences does not entail knowledge of the quadrature training symbols.
Hence, there exist a need for a method and a system that is able to efficiently demodulate a signal modulated using a hybrid quadrature modulation. In particular there exist a need for a demodulator that efficiently can demodulate an α-QPSK modulated signal.